Hi. When you give somebody rep you give them the full amount you can, there's no way to control it. The amount of rep you dispense in one shot is related to your own rep level, your length of membership, etc. I think the details are on here somewhere. Look for a thread called something like "What is rep and how do I use it?"
Edit: you can see your own rep, or someone else's, by looking at the dots under their username. The more dots, the more rep. Grey dots are no rep, I think. It used to tell you how many points if you rolled over it but I think it might just show a "Bill's really cool" type of tag based on your rank. You can see your actual points if you look in your Settings page, I think.